Photonic crystal fibers, in the following referred to as PCF or microstructured optical fibers, are fibers having a core surrounded by a cladding region having a plurality of inclusions (sometimes called cladding features or microstructures) arranged in a background material, typically in a regular array. The inclusion may be gas, liquid, or solid inclusion. In principle the inclusions could be void, but in practice the voids will normally comprise some gas molecules.
Fibers of this types are well known in the art and are for example described in US 2012/195554, U.S. Pat. No. 8,406,594, US 2011/116283 and US 2012/195554.
The microstructured fiber may for example be of silica glass. Other materials may be added to the silica glass in order to alter the refractive index thereof or to provide effects, such as amplification of light, sensitivity, etc.
The center-to-center spacing between the cladding inclusions is defined as the pitch (A). The PCFs are usually at least partly characterized by the size of the core and the ratio of the size of the inclusions to their spacing or pitch (A). By tailoring the size and pitch of the cladding inclusions, the zero dispersion wavelength (ZDW) of the fiber may be tailored.
Photonic crystal fibers are in general suitable for use in high power light sources. Guiding of relatively high powers in an optical fiber may have relevance for several commercial applications such as such as guiding of surgical and/or therapeutic light, optical sensing, and materials processing. Among such applications is transport of optical energy and utilizing of non-linear effects in the fiber which are commonly more pronounced with higher optical power inside the fiber. The optical power may be continuous wave (CW), pulsed or a mixture thereof. High optical power inside a fiber may be particularly pronounced with pulsed light where a high peak power may be obtainable even while having a relatively modest average power.
One limitation of the average power/spectral density carried by an optical fiber is the damage threshold of the fiber. In particular where the PCF is applied for supercontinuum generation where high power light is fed to the PCF via a launching end (sometimes called an input end) of the PCF, it has been found that the PCF degrades over time in dependence on the peak power of the fed light. Further it has been found that a fiber section adjacent to or close to the launching end is more exposed to degradation than longer from the launching end.
U.S. Pat. No. 8,145,023 describes a method of alleviating the degradation caused by the high power light fed to the PCF by loading the core material and optionally the cladding material with hydrogen and/or deuterium. This loading was found to result in some increase in the lifetime of the fiber. In US 2011/116283 the method was further improved by subjecting the PCF to an annealing and/or to a high power irradiation after the hydrogen and/or deuterium loading.